MT63 has the facility for a secondary channel running simultaneously alongside the main channel. This can be put to a variety of uses, such as the generation of a continuous identification or beacon.
The secondary channel is not a prime function of the mode and therefore some software provides for it and others do not. The option to transfer binary files, such as higher-level documents or spreadsheets, is similarly at the whim of the programmer.
MT63 is perhaps the most elaborate user of error correction techniques. It uses a Walsh function that spreads the data bits of each character across all 64 of the tones of the signal spectrum and simultaneously repeats the information over a period of 64 symbols within any one tone. This takes 6.4 seconds.
The combination results in superb impulse noise rejection. At the same time, in the frequency domain, significant portions of the signal can be masked by unwanted noise or other transmissions without any noticeable effect on successful reception.
Transmission speed is good because there are so many individual tones to describe the information, while the individual symbol rate per tone can remain slow (which is good protection against ionospheric disturbances).
Tuning of MT63 modes is not all that critical. This is because the mode can use Forward Error Correction techniques to examine different combinations of the 64 tones that calculate the correct location within the spectrum. As an example, MT63-1K will still work if the decoder is off tune by as much as 100Hz. MT63-2K is even less exacting, with an error of 250Hz being tolerated.
Although it can be a little tricky to identify by ear or eye, the mode has a generous tolerance to tuning inaccuracies and its immunity to impulse noise is second to none.
While only occupying a modest amount of spectrum, the error correction and time interleaving combine to recover the data from the most marginal signals, a full 10dB lower in the noise than MT63 can manage.
One of the compromises with MFSK16 is the heavy burden of 100% power demand on the transmitter. It is also notoriously difficult to tune when barely visible on the waterfall display and very stable equipment is a prerequisite for successful operation.
This often means that little improvement is noticed in practice. The PSK modes are very easy to tune in and work reliably on modest computers.
Another great advantage of BPSK31 in particular is that it is the most well known and easily understood of the data modes.
Not much exploration has yet taken place into some of the newer or more obscure variants of these data modes. For example, PSK125 is similar to PSK31 but by using two sidebands of 125Hz the data rate is dramatically increased. The greater bandwidth also allows the introduction of improved error correction. There is also PSK250, operating at a faster rate again.
However, the error correction employed does not approach that of MT63 and, as with all PSK modes, success is limited by the need for stable propagation conditions and, as the data rate climbs, increasingly powerful computing facilities.
Unlike most HF modes where a character can be lost or changed into something else, by a single noise burst, MT63 is inherently very robust, because each character is spread over many tones (to avoid interference such as other radio transmissions) and over several seconds (to avoid bursts of noise, such as lightning).
MT63's COFDM like properties
Despite the low data rate, good text speed is maintained, because the text is sent on many tones at once. The system runs at several different speeds, which can be chosen to suit conditions, but 100 WPM is typical.
MT63 sounds unusual, (it sounds like a roaring noise) but the performance is spectacular. There is no connection process, as in AMTOR, Packet or PACTOR. Some users maintain that under poor propagation conditions (namly excessive fading) MT63 works better than either PACTOR II or Clover. Under good conditions the performance advantages are less obvious.
In a "long interleave" option, the spreading is over 64 symbols (6.4 sec), with consequent improvement in resistance to impulse and periodic interference, but of course double the time taken for the data to "trickle through" the Walsh encoder and decoder pipeline.
Changeover from transmit to receive and vice-versa is however considerably slower than most modes. It therefore requires some skill and patience to "break in" on a conversation.
Also, because of the delay through the error correction and interleaving processes, it is not possible have quick turnaround "slick" conversations. In other words, operation is clumsy.
Another problem with the reception of MT63 is the fact that correctly decoded text can take up to 15 seconds to appear on the screen. A clue to synchronized reception can sometimes be gleaned if the software has a digital squelch facility. If the squelch is set up to be closed when no MT63 signal is present, it can often be seen to open as soon as it finds MT63 and ideally the text will follow quite a number of seconds later! Another clue to satisfactory reception prior to the actual appearance of the text is that the generation of large volumes of unwanted characters tends to cease when receiver synchronization has been achieved. As with other multi-tone modes where many sine waves exists side by side, the transmission chain for MT63 must be made as linear as possible.
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The same temporal interleaving techniques could be used with turbo codes, but as a general principle only short length interleaving should be used with turbo codes.